Mechanical disc brakes incorporating power screws have the advantage of efficiently applying large braking force to the friction pad. Such disc brakes of the limited area or spot-type have application for use in truck, and tractor-trailer combinations which already incorporate an air compressor for operation of the brakes and the suspension. Commonly, such air systems are regulated to about 90-100 psi with a maximum output of about 120 psi. However, hydraulic brakes operate with hydraulic pressures up to 1,800 psi or more, and accordingly, the force output of the braking system in an air-operated disc brake must be comparable to the ranges of forces which can be obtained with an 1,800 psi hydraulic system. Typically, piston areas in such hydraulic systems may be around fifteen square inches with pads of around twenty square inches per pad. The total piston area may be distributed among two or more pistons.
In adapting disc brakes to the pneumatic systems now existing and in common use on truck and tractor-trailer combinations, it is desired to make use of the standard air actuator chambers which are now commonly in use. These air chambers provide from twelve to thirty-six inches of effective area exposed to the pneumatic pressure, and generally are limited to a relatively short working stroke, such as about two and one-fourth inches.
Due to the limited stroke available from conventional air actuators or chambers, it has been found either necessary to limit the running clearances of the brake to a minimum, reduce the lever arm length, or otherwise reduce the mechanical advantage in order to insure against running out of stroke. However, it is desirable to maintain an adequate running clearance particularly in large heavy-duty brakes which are used on tractors or semi-trailers since the provision of an adequate running clearance enhances the cooling of the brake, and assures against a dragging brake condition.
It is known to use ball and ramp arrangements in mechanically-operated disc brakes, as shown in U.S. Pat. Nos. 3,059,731, 3,110,364, 3,115,217 and 3,638,763. It is also known to provide the ramp with varying slope characteristics to obtain a desired rate of force application. In such brakes, the reactive forces must be transmitted from one ramp member to the other through the balls. While ball and ramp actuators have had some success, they are severely limited by the high pressures and wear which occur on the ball ramp in high force applications. Further, such arrangements are inherently costly to manufacture.
Lead screws have also been suggested as a force applying mechanism in disc brakes, as shown for example in U.S. Pat. Nos. 1,612,356 and 3,647,015. The power screw has the advantage of simplicity, high efficiency and long service life.
It has also been known for many years in hydraulic brakes to use compound cylinders or pistons to take up the running clearance rapidly and thereafter to apply a higher force as shown, for example, in U.S. Pat. No. 3,650,364.